Magnet systems incorporating superconductive coils, the current in which is ramped up and down (pulsed), have application in such diverse fields as material separation and energy storage. For example, a magnetic separator can be employed to process kaolin (a white clay) that is used to provide shiny veneer on paper. Kaolin as mined has trace quantities of titanium dioxide (TiO.sub.2) which gives the kaolin a yellow cast. The (TiO.sub.2) is paramagnetic whereas the kaolin is nonmagnetic. The granular kaolin to be processed is mixed with water to form a slurry which is directed to the working bore of the magnetic separator packed with stainless steel wool which, in the presence of the intense magnetic field provided by the coil, attracts and holds the titanium dioxide. With time, the steel wool becomes saturated with the TiO.sub.2 and the coil current must be ramped down to permit flushing of the TiO.sub.2 from the steel wool. The current must then be ramped up again to effect separation. Ramping times can be in the order of 60 seconds and the current in the coil may be in the order of 1000 amperes.
Conventional superconductive material has a critical temperature only slightly above the boiling point of liquid helium. Magnet systems typically maintain the coil in its superconductive state by placing the coil in a helium containment vessel made of stainless steel to provide a vacuum seal and having sufficient thickness to withstand the mechanical forces applied to the vessel. Such a helium vessel operates satisfactorily in magnet systems used for magnetic resonance imaging (MRI) because, in that application, the coil current is constant to provide a continuous homogeneous magnetic field. In a pulsed magnet system, with the magnet field being frequently collapsed and then reestablished, significant resistive heating occurs in the helium vessel due to the generation of eddy currents. To maintain the coil in its superconductive state, large capacity refrigeration equipment is required, and the major cost of operation of a pulsed magnet system can be the cost of running the refrigeration equipment.
U.S. Pat. No. 3,360,692 to Kafka is directed to a magnetic device for providing high-intensity fields of short duration for use in, for example, plasma confinement. With reference to FIGS. 3 and 4 of that patent, a superconductor 24 is embedded between two adjacent metallic jackets 15, each of which has an axial slit 25 to prevent the jacket from forming a short circuit. The conductor 24 is spaced from the jacket by layers of insulation 26. The slits 25 also serve as conduits for coolant, see Col. 3, 1. 72 through Col. 4, 1. 59.
U.S. Pat. No. 4,609,109 to Good shows a magnetic separator. The thrust of the patent is that certain economies can be achieved using oval shaped coils as opposed to circular coils, Col. 2, 11. 24-60.
U.S. Pat. No. 4,702,825 to Selvaggi et al. teaches a magnet separator including a superconductive coil. The current is ramped up and down to permit periodic flushing of trapped containments from the bore of the magnetic which accepts canisters packed with a matrix of magnetic stainless steel wool. The magnet is supported at both ends so that it will take side loads as well as compressive loads.
U.S. Pat. No. 4,707,676 to Saitou et al. shows a magnet system in which the outer peripheral portion of the cryostat is thickened so that it serves as a magnetic shield.
U.S. Pat. No. 4,768,008 to Purcell et al. illustrates a magnet system in which the inner wall of the vacuum vessel has a relatively thin metallic portion backed up by a non-metallic support portion with the result being that the first wall is relatively transparent to the gradient field used in MRI.